Magnetotransport properties of rare earth element modified carbon nanotubes

Ncube, Siphephile
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Functionalization and filling of carbon nanotubes has been tailored over years to modify the exceptional properties of the 1-dimensional (1D) conductor for magnetic properties based applications. Hence such a system exploits the spin and charge property of the electron, analogous to a quantum conductor coupled to magnetic impurities which poses an interesting scenario for the study of Kondo physics and related phenomena. A study of the low temperature electronic transport and magnetic properties of carbon nanotubes modified with gadolinium derivatives is presented in this thesis. The methods of modification used are chemical functionalization and capillary filling. The presence of gadolinium in the nanostructures extends the functionality of the nanotubes from conventional electronics to spintronics. Filled and functionalized multiwalled carbon nanotubes are characterized as well as filled double walled carbon nanotubes. This system gives a chance to study the interaction of a ballistic conductor with magnetic impurities. Multiwalled carbon nanotubes functionalized with a gadolinium based supramolecular complex show enhanced magnetic properties and unexpected electronic behaviour that has not been observed in this material before. A newly developed synthesis technique has been employed for the synthesis and it is found that the functionalization method of the nanocomposite enhances the strength of magnetic interaction leading to a large effective moment of 15.79 μB and non-superparamagnetic behaviour unlike what has been previously reported. Saturating resistance at low temperatures is fitted with the numerical renormalization group formula verifying the Kondo effect for magnetic impurities on a metallic electron system. Magnetoresistance shows that devices fabricated from aligned gadolinium functionalized MWNTs exhibit spin-valve switching behaviour of up to 8%. The electronic transport properties of MWNTs filled with GdCl3 nanomagnets clearly shows the co-existence of Kondo correlation and cotunelling within the superparamagnetic limit. The Fermi liquid description of the Kondo effect and the interpolation scheme are fitted to the resistance-temperature dependence yielding the onset of the Kondo scattering temperature and a Kondo temperature for this nanocomposite, respectively. Cotunneling of conduction electrons inhibiting a Kondo type interaction has been verified from the exponential decay of the intensity of the fano shaped non zero bias anomalous conductance peaks which also show strong resonant features observed only in GdCl3 filled MWNT devices. Hence these features are explained in terms of magnetic coherence and spin-flip effects along with the competition between the Kondo effect and co-tunneling. The properties of doublewalled carbon naotubes filled with GdCl3 are also presented. They show superparamagnetic behaviour and zero bias anomalies similar to what was observed in Gd filled MWNTs. This work is the first on such lanthanide modified CNT hybrid bundle devices. The study raises a new possibility of tailoring magnetic interactions for spintronic applications in carbon nanotube systems. It highlights the possibility of enhancing magnetic interactions in carbon systems through chemical modification. Furthermore, the study demonstrates the rich physics that might be useful for developing spin-based quantum computing elements based on 1D channels.
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy. University of the Witwatersrand Johannesburg
Ncube, Siphephile (2018) Magnetotransport properties of rare earth element modified carbon nanotubes, University of the Witwatersrand, Johannesburg, <>